I just ran Francis' sample fex. The datetime fields are not in actual excel date/time format, but they displayed properly. For example DTGC displays as "2005/12/29 4:45:00PM". Excel sees it as general text, not a date/time field. I assume the problem is that, while Excel displays the field properly, Excel sees it at general text instead of date/time.

Just for the record, Excel see the QYY as general text and the YY as a zip code, all of the others, Excel sees as date fields.

SUM
MIN.DET_FIRSTDETECTDATETIME/HYYMDS
MAX.DET_LASTDETECTDATETIME/HYYMDS
BY SITE
BY TAGCODE
ON TABLE PCHOLD FORMAT EXL2K
END

with output to EXL2K and was able to do perform date math in Excel without reformatting the date columns. I subtracted LastDetectDateTime (D1) from FirstDetectDateTime (C1)and got the correct answer (in hours).

It stays in 'General' format. Interestingly, Excel treats the columns as if they were date/time format if you use them in a date function. I also tried 'ON TABLE PCHOLD FORMAT EXL07'. It also created the columns as 'General'.